Helpmate in 2: (a) diagram (b) move d4 to e4

A mate of the black king in two moves cannot be orchestrated with it on d4; the monarch must relocate — but to where? The diagram yields to an uncanny sequence: 1.Qg5+ Bf4 2.Kc5 Be3# (MM), showcasing the subtle effect known as the Anticipatory Self-pin. The bQ blocks the wR on the fourth rank, freeing c4 for the bK. Then, as the bK steps onto this square, the wB crowns the line with a Pelle move to deliver mate.

The solution to part (b), with Kd4 shifted to e4, is perfectly analogous: 1.Qc7+ Rc5 2.Kf4 Rc4# (MM). This time, the bQ is pinned along a different line, with the wB and wR exchanging roles — the former now pins, and the latter delivers mate.

Flawlessly harmonious!